For most people it is a matter of training.
Yeah, some people have a weaker gag reflex than others, so it’s easier for them, but at the end of the day most people need to train the ability to swallow.
The basics are: Slowly going deeper, bigger and lasting longer. Allow your body to adapt to the feeling without instantly “panicking” (= gagging). It takes time. The key is to relax, not to tense up.
If you tested your ability with a toothbrush already, you can make that a routine. Do it whenever you brush your teeth and slowly get used to the feeling.
Once you got somewhat used to that, you can move on to bigger stuff and try to take it deeper. I advise against trying to “force” it and go big too quickly, no matter how enticing it may be. You may end up throwing up.
As a safety precaution make sure that whatever you train with has something you can firmly grab onto or a flared base in the case of a dildo.